diff --git a/libc/Android.mk b/libc/Android.mk
index 2609644..85a58e7 100644
--- a/libc/Android.mk
+++ b/libc/Android.mk
@@ -464,9 +464,7 @@
     upstream-openbsd/lib/libc/stdio/fputws.c \
     upstream-openbsd/lib/libc/stdio/freopen.c \
     upstream-openbsd/lib/libc/stdio/fscanf.c \
-    upstream-openbsd/lib/libc/stdio/fseek.c \
     upstream-openbsd/lib/libc/stdio/fsetpos.c \
-    upstream-openbsd/lib/libc/stdio/ftell.c \
     upstream-openbsd/lib/libc/stdio/funopen.c \
     upstream-openbsd/lib/libc/stdio/fvwrite.c \
     upstream-openbsd/lib/libc/stdio/fwalk.c \
diff --git a/libc/include/stdio.h b/libc/include/stdio.h
index fd653d9..268db78 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -73,9 +73,9 @@
 #define	__SMBF	0x0080		/* _buf is from malloc */
 #define	__SAPP	0x0100		/* fdopen()ed in append mode */
 #define	__SSTR	0x0200		/* this is an sprintf/snprintf string */
-#define	__SOPT	0x0400		/* do fseek() optimization */
-#define	__SNPT	0x0800		/* do not do fseek() optimization */
-#define	__SOFF	0x1000		/* set iff _offset is in fact correct */
+/* #define __SOPT 0x0400 --- historical (do fseek() optimization). */
+/* #define __SNPT 0x0800 --- historical (do not do fseek() optimization). */
+/* #define __SOFF 0x1000 --- historical (set iff _offset is in fact correct). */
 #define	__SMOD	0x2000		/* true => fgetln modified _p text */
 #define	__SALC	0x4000		/* allocate string space dynamically */
 #define	__SIGN	0x8000		/* ignore this file in _fwalk */
diff --git a/libc/stdio/local.h b/libc/stdio/local.h
index a15bddf..7188c74 100644
--- a/libc/stdio/local.h
+++ b/libc/stdio/local.h
@@ -92,7 +92,6 @@
 
 	/* Unix stdio files get aligned to block boundaries on fseek() */
 	int	_blksize;	/* stat.st_blksize (may be != _bf._size) */
-	fpos_t	_offset;	/* current lseek offset */
 };
 
 /*
@@ -112,6 +111,10 @@
   bool _caller_handles_locking;
 };
 
+// TODO: remove remaining references to these obsolete flags.
+#define __SNPT 0
+#define __SOPT 0
+
 #if defined(__cplusplus)
 #define _EXT(fp) reinterpret_cast<__sfileext*>((fp)->_ext._base)
 #else
diff --git a/libc/stdio/stdio.cpp b/libc/stdio/stdio.cpp
index 1ea3ef5..1066d7f 100644
--- a/libc/stdio/stdio.cpp
+++ b/libc/stdio/stdio.cpp
@@ -35,6 +35,7 @@
 
 #include <errno.h>
 #include <fcntl.h>
+#include <limits.h>
 #include <stdlib.h>
 #include <string.h>
 #include <sys/param.h>
@@ -51,7 +52,7 @@
 
 #define	std(flags, file) \
 	{0,0,0,flags,file,{0,0},0,__sF+file,__sclose,__sread,__sseek,__swrite, \
-	    {(unsigned char *)(__sFext+file), 0},NULL,0,{0},{0},{0,0},0,0}
+	    {(unsigned char *)(__sFext+file), 0},NULL,0,{0},{0},{0,0},0}
 
 _THREAD_PRIVATE_MUTEX(__sfp_mutex);
 
@@ -85,25 +86,38 @@
 struct glue __sglue = { NULL, 3, __sF };
 static struct glue* lastglue = &__sglue;
 
+class ScopedFileLock {
+ public:
+  ScopedFileLock(FILE* fp) : fp_(fp) {
+    FLOCKFILE(fp_);
+  }
+  ~ScopedFileLock() {
+    FUNLOCKFILE(fp_);
+  }
+
+ private:
+  FILE* fp_;
+};
+
 static glue* moreglue(int n) {
-    static FILE empty;
+  static FILE empty;
 
-    char* data = new char[sizeof(glue) + ALIGNBYTES + n * sizeof(FILE) + n * sizeof(__sfileext)];
-    if (data == nullptr) return nullptr;
+  char* data = new char[sizeof(glue) + ALIGNBYTES + n * sizeof(FILE) + n * sizeof(__sfileext)];
+  if (data == nullptr) return nullptr;
 
-    glue* g = reinterpret_cast<glue*>(data);
-    FILE* p = reinterpret_cast<FILE*>(ALIGN(data + sizeof(*g)));
-    __sfileext* pext = reinterpret_cast<__sfileext*>(ALIGN(data + sizeof(*g)) + n * sizeof(FILE));
-    g->next = NULL;
-    g->niobs = n;
-    g->iobs = p;
-    while (--n >= 0) {
-        *p = empty;
-        _FILEEXT_SETUP(p, pext);
-        p++;
-        pext++;
-    }
-    return g;
+  glue* g = reinterpret_cast<glue*>(data);
+  FILE* p = reinterpret_cast<FILE*>(ALIGN(data + sizeof(*g)));
+  __sfileext* pext = reinterpret_cast<__sfileext*>(ALIGN(data + sizeof(*g)) + n * sizeof(FILE));
+  g->next = NULL;
+  g->niobs = n;
+  g->iobs = p;
+  while (--n >= 0) {
+    *p = empty;
+    _FILEEXT_SETUP(p, pext);
+    p++;
+    pext++;
+  }
+  return g;
 }
 
 /*
@@ -147,81 +161,155 @@
 }
 
 extern "C" __LIBC_HIDDEN__ void __libc_stdio_cleanup(void) {
-	/* (void) _fwalk(fclose); */
-	(void) _fwalk(__sflush);		/* `cheating' */
+  // Equivalent to fflush(nullptr), but without all the locking since we're shutting down anyway.
+  _fwalk(__sflush);
 }
 
 int fclose(FILE* fp) {
-    if (fp->_flags == 0) {
-        // Already freed!
-        errno = EBADF;
-        return EOF;
-    }
+  if (fp->_flags == 0) {
+    // Already freed!
+    errno = EBADF;
+    return EOF;
+  }
 
-    FLOCKFILE(fp);
-    WCIO_FREE(fp);
-    int r = fp->_flags & __SWR ? __sflush(fp) : 0;
-    if (fp->_close != NULL && (*fp->_close)(fp->_cookie) < 0) {
-        r = EOF;
-    }
-    if (fp->_flags & __SMBF) free(fp->_bf._base);
-    if (HASUB(fp)) FREEUB(fp);
-    if (HASLB(fp)) FREELB(fp);
+  ScopedFileLock sfl(fp);
+  WCIO_FREE(fp);
+  int r = fp->_flags & __SWR ? __sflush(fp) : 0;
+  if (fp->_close != NULL && (*fp->_close)(fp->_cookie) < 0) {
+    r = EOF;
+  }
+  if (fp->_flags & __SMBF) free(fp->_bf._base);
+  if (HASUB(fp)) FREEUB(fp);
+  if (HASLB(fp)) FREELB(fp);
 
-    // Poison this FILE so accesses after fclose will be obvious.
-    fp->_file = -1;
-    fp->_r = fp->_w = 0;
+  // Poison this FILE so accesses after fclose will be obvious.
+  fp->_file = -1;
+  fp->_r = fp->_w = 0;
 
-    // Release this FILE for reuse.
-    fp->_flags = 0;
-    FUNLOCKFILE(fp);
-    return (r);
+  // Release this FILE for reuse.
+  fp->_flags = 0;
+  return r;
 }
 
 int fileno(FILE* fp) {
-    FLOCKFILE(fp);
-    int result = fileno_unlocked(fp);
-    FUNLOCKFILE(fp);
-    return result;
+  ScopedFileLock sfl(fp);
+  return fileno_unlocked(fp);
 }
 
-// Small standard I/O/seek/close functions.
-// These maintain the `known seek offset' for seek optimisation.
 int __sread(void* cookie, char* buf, int n) {
-    FILE* fp = reinterpret_cast<FILE*>(cookie);
-    int ret = TEMP_FAILURE_RETRY(read(fp->_file, buf, n));
-    // If the read succeeded, update the current offset.
-    if (ret >= 0) {
-        fp->_offset += ret;
-    } else {
-        fp->_flags &= ~__SOFF; // Paranoia.
-    }
-    return ret;
+  FILE* fp = reinterpret_cast<FILE*>(cookie);
+  return TEMP_FAILURE_RETRY(read(fp->_file, buf, n));
 }
 
 int __swrite(void* cookie, const char* buf, int n) {
-    FILE* fp = reinterpret_cast<FILE*>(cookie);
-    if (fp->_flags & __SAPP) {
-        (void) TEMP_FAILURE_RETRY(lseek(fp->_file, 0, SEEK_END));
-    }
-    fp->_flags &= ~__SOFF; // In case FAPPEND mode is set.
-    return TEMP_FAILURE_RETRY(write(fp->_file, buf, n));
+  FILE* fp = reinterpret_cast<FILE*>(cookie);
+  if (fp->_flags & __SAPP) {
+    // The FILE* is in append mode, but the underlying fd doesn't have O_APPEND set.
+    // We need to seek manually.
+    // TODO: can we use fcntl(2) to set O_APPEND in fdopen(3) instead?
+    TEMP_FAILURE_RETRY(lseek64(fp->_file, 0, SEEK_END));
+  }
+  return TEMP_FAILURE_RETRY(write(fp->_file, buf, n));
 }
 
 // TODO: _FILE_OFFSET_BITS=64.
 fpos_t __sseek(void* cookie, fpos_t offset, int whence) {
-    FILE* fp = reinterpret_cast<FILE*>(cookie);
-    off_t ret = TEMP_FAILURE_RETRY(lseek(fp->_file, offset, whence));
-    if (ret == -1) {
-        fp->_flags &= ~__SOFF;
-    } else {
-        fp->_flags |= __SOFF;
-        fp->_offset = ret;
-    }
-    return ret;
+  FILE* fp = reinterpret_cast<FILE*>(cookie);
+  return TEMP_FAILURE_RETRY(lseek(fp->_file, offset, whence));
 }
 
 int __sclose(void* cookie) {
-    FILE* fp = reinterpret_cast<FILE*>(cookie);
-    return close(fp->_file);
+  FILE* fp = reinterpret_cast<FILE*>(cookie);
+  return close(fp->_file);
+}
+
+static bool __file_is_seekable(FILE* fp) {
+  if (fp->_seek == nullptr) {
+    errno = ESPIPE;  // Historic practice.
+    return false;
+  }
+  return true;
+}
+
+// TODO: _FILE_OFFSET_BITS=64.
+static off_t __ftello_unlocked(FILE* fp) {
+  if (!__file_is_seekable(fp)) return -1;
+
+  // Find offset of underlying I/O object, then
+  // adjust for buffered bytes.
+  __sflush(fp);  // May adjust seek offset on append stream.
+  fpos_t result = (*fp->_seek)(fp->_cookie, 0, SEEK_CUR);
+  if (result == -1) {
+    return -1;
+  }
+
+  if (fp->_flags & __SRD) {
+    // Reading.  Any unread characters (including
+    // those from ungetc) cause the position to be
+    // smaller than that in the underlying object.
+    result -= fp->_r;
+    if (HASUB(fp)) result -= fp->_ur;
+  } else if (fp->_flags & __SWR && fp->_p != NULL) {
+    // Writing.  Any buffered characters cause the
+    // position to be greater than that in the
+    // underlying object.
+    result += fp->_p - fp->_bf._base;
+  }
+  return result;
+}
+
+int fseeko(FILE* fp, off_t offset, int whence) {
+  ScopedFileLock sfl(fp);
+
+  if (!__file_is_seekable(fp)) return -1;
+
+  // Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
+  // After this, whence is either SEEK_SET or SEEK_END.
+  if (whence == SEEK_CUR) {
+    fpos_t current_offset = __ftello_unlocked(fp);
+    if (current_offset == -1) {
+      return EOF;
+    }
+    offset += current_offset;
+    whence = SEEK_SET;
+  } else if (whence != SEEK_SET && whence != SEEK_END) {
+    errno = EINVAL;
+    return EOF;
+  }
+
+  if (fp->_bf._base == NULL) __smakebuf(fp);
+
+  // Flush unwritten data and attempt the seek.
+  if (__sflush(fp) || (*fp->_seek)(fp->_cookie, offset, whence) == -1) {
+    return EOF;
+  }
+
+  // Success: clear EOF indicator and discard ungetc() data.
+  if (HASUB(fp)) FREEUB(fp);
+  fp->_p = fp->_bf._base;
+  fp->_r = 0;
+  /* fp->_w = 0; */	/* unnecessary (I think...) */
+  fp->_flags &= ~__SEOF;
+  return 0;
+}
+
+// TODO: _FILE_OFFSET_BITS=64.
+int fseek(FILE* fp, long offset, int whence) {
+  return fseeko(fp, offset, whence);
+}
+
+// TODO: _FILE_OFFSET_BITS=64.
+off_t ftello(FILE* fp) {
+  ScopedFileLock sfl(fp);
+  return __ftello_unlocked(fp);
+}
+
+// TODO: _FILE_OFFSET_BITS=64
+long ftell(FILE* fp) {
+  off_t offset = ftello(fp);
+  if (offset > LONG_MAX) {
+    errno = EOVERFLOW;
+    return -1;
+  }
+  return offset;
 }
diff --git a/libc/upstream-openbsd/lib/libc/stdio/fseek.c b/libc/upstream-openbsd/lib/libc/stdio/fseek.c
deleted file mode 100644
index cdd40b6..0000000
--- a/libc/upstream-openbsd/lib/libc/stdio/fseek.c
+++ /dev/null
@@ -1,251 +0,0 @@
-/*	$OpenBSD: fseek.c,v 1.11 2012/05/21 22:24:19 matthew Exp $ */
-/*-
- * Copyright (c) 1990, 1993
- *	The Regents of the University of California.  All rights reserved.
- *
- * This code is derived from software contributed to Berkeley by
- * Chris Torek.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- * 3. Neither the name of the University nor the names of its contributors
- *    may be used to endorse or promote products derived from this software
- *    without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <fcntl.h>
-#include <stdio.h>
-#include <stdlib.h>
-#include <errno.h>
-#include "local.h"
-
-#define	POS_ERR	(-(fpos_t)1)
-
-/*
- * Seek the given file to the given offset.
- * `Whence' must be one of the three SEEK_* macros.
- */
-int
-fseeko(FILE *fp, off_t offset, int whence)
-{
-	fpos_t (*seekfn)(void *, fpos_t, int);
-	fpos_t target, curoff;
-	size_t n;
-	struct stat st;
-	int havepos;
-
-	/* make sure stdio is set up */
-	if (!__sdidinit)
-		__sinit();
-
-	/*
-	 * Have to be able to seek.
-	 */
-	if ((seekfn = fp->_seek) == NULL) {
-		errno = ESPIPE;			/* historic practice */
-		return (EOF);
-	}
-
-	/*
-	 * Change any SEEK_CUR to SEEK_SET, and check `whence' argument.
-	 * After this, whence is either SEEK_SET or SEEK_END.
-	 */
-	FLOCKFILE(fp);
-	switch (whence) {
-
-	case SEEK_CUR:
-		/*
-		 * In order to seek relative to the current stream offset,
-		 * we have to first find the current stream offset a la
-		 * ftell (see ftell for details).
-		 */
-		__sflush(fp);	/* may adjust seek offset on append stream */
-		if (fp->_flags & __SOFF)
-			curoff = fp->_offset;
-		else {
-			curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
-			if (curoff == (fpos_t)-1) {
-				FUNLOCKFILE(fp);
-				return (EOF);
-			}
-		}
-		if (fp->_flags & __SRD) {
-			curoff -= fp->_r;
-			if (HASUB(fp))
-				curoff -= fp->_ur;
-		} else if (fp->_flags & __SWR && fp->_p != NULL)
-			curoff += fp->_p - fp->_bf._base;
-
-		offset += curoff;
-		whence = SEEK_SET;
-		havepos = 1;
-		break;
-
-	case SEEK_SET:
-	case SEEK_END:
-		curoff = 0;		/* XXX just to keep gcc quiet */
-		havepos = 0;
-		break;
-
-	default:
-		FUNLOCKFILE(fp);
-		errno = EINVAL;
-		return (EOF);
-	}
-
-	/*
-	 * Can only optimise if:
-	 *	reading (and not reading-and-writing);
-	 *	not unbuffered; and
-	 *	this is a `regular' Unix file (and hence seekfn==__sseek).
-	 * We must check __NBF first, because it is possible to have __NBF
-	 * and __SOPT both set.
-	 */
-	if (fp->_bf._base == NULL)
-		__smakebuf(fp);
-	if (fp->_flags & (__SWR | __SRW | __SNBF | __SNPT))
-		goto dumb;
-	if ((fp->_flags & __SOPT) == 0) {
-		if (seekfn != __sseek ||
-		    fp->_file < 0 || fstat(fp->_file, &st) ||
-		    (st.st_mode & S_IFMT) != S_IFREG) {
-			fp->_flags |= __SNPT;
-			goto dumb;
-		}
-		fp->_blksize = st.st_blksize;
-		fp->_flags |= __SOPT;
-	}
-
-	/*
-	 * We are reading; we can try to optimise.
-	 * Figure out where we are going and where we are now.
-	 */
-	if (whence == SEEK_SET)
-		target = offset;
-	else {
-		if (fstat(fp->_file, &st))
-			goto dumb;
-		target = st.st_size + offset;
-	}
-
-	if (!havepos) {
-		if (fp->_flags & __SOFF)
-			curoff = fp->_offset;
-		else {
-			curoff = (*seekfn)(fp->_cookie, (fpos_t)0, SEEK_CUR);
-			if (curoff == POS_ERR)
-				goto dumb;
-		}
-		curoff -= fp->_r;
-		if (HASUB(fp))
-			curoff -= fp->_ur;
-	}
-
-	/*
-	 * Compute the number of bytes in the input buffer (pretending
-	 * that any ungetc() input has been discarded).  Adjust current
-	 * offset backwards by this count so that it represents the
-	 * file offset for the first byte in the current input buffer.
-	 */
-	if (HASUB(fp)) {
-		curoff += fp->_r;	/* kill off ungetc */
-		n = fp->_up - fp->_bf._base;
-		curoff -= n;
-		n += fp->_ur;
-	} else {
-		n = fp->_p - fp->_bf._base;
-		curoff -= n;
-		n += fp->_r;
-	}
-
-	/*
-	 * If the target offset is within the current buffer,
-	 * simply adjust the pointers, clear EOF, undo ungetc(),
-	 * and return.  (If the buffer was modified, we have to
-	 * skip this; see fgetln.c.)
-	 */
-	if ((fp->_flags & __SMOD) == 0 &&
-	    target >= curoff && target < curoff + n) {
-		int o = target - curoff;
-
-		fp->_p = fp->_bf._base + o;
-		fp->_r = n - o;
-		if (HASUB(fp))
-			FREEUB(fp);
-		fp->_flags &= ~__SEOF;
-		FUNLOCKFILE(fp);
-		return (0);
-	}
-
-	/*
-	 * The place we want to get to is not within the current buffer,
-	 * but we can still be kind to the kernel copyout mechanism.
-	 * By aligning the file offset to a block boundary, we can let
-	 * the kernel use the VM hardware to map pages instead of
-	 * copying bytes laboriously.  Using a block boundary also
-	 * ensures that we only read one block, rather than two.
-	 */
-	curoff = target & ~(fp->_blksize - 1);
-	if ((*seekfn)(fp->_cookie, curoff, SEEK_SET) == POS_ERR)
-		goto dumb;
-	fp->_r = 0;
- 	fp->_p = fp->_bf._base;
-	if (HASUB(fp))
-		FREEUB(fp);
-	fp->_flags &= ~__SEOF;
-	n = target - curoff;
-	if (n) {
-		if (__srefill(fp) || fp->_r < n)
-			goto dumb;
-		fp->_p += n;
-		fp->_r -= n;
-	}
-	FUNLOCKFILE(fp);
-	return (0);
-
-	/*
-	 * We get here if we cannot optimise the seek ... just
-	 * do it.  Allow the seek function to change fp->_bf._base.
-	 */
-dumb:
-	if (__sflush(fp) ||
-	    (*seekfn)(fp->_cookie, (fpos_t)offset, whence) == POS_ERR) {
-		FUNLOCKFILE(fp);
-		return (EOF);
-	}
-	/* success: clear EOF indicator and discard ungetc() data */
-	if (HASUB(fp))
-		FREEUB(fp);
-	fp->_p = fp->_bf._base;
-	fp->_r = 0;
-	/* fp->_w = 0; */	/* unnecessary (I think...) */
-	fp->_flags &= ~__SEOF;
-	FUNLOCKFILE(fp);
-	return (0);
-}
-
-int
-fseek(FILE *fp, long offset, int whence)
-{
-	return (fseeko(fp, offset, whence));
-}
diff --git a/libc/upstream-openbsd/lib/libc/stdio/ftell.c b/libc/upstream-openbsd/lib/libc/stdio/ftell.c
deleted file mode 100644
index 0a2016c..0000000
--- a/libc/upstream-openbsd/lib/libc/stdio/ftell.c
+++ /dev/null
@@ -1,96 +0,0 @@
-/*	$OpenBSD: ftell.c,v 1.10 2012/05/21 22:24:19 matthew Exp $ */
-/*-
- * Copyright (c) 1990, 1993
- *	The Regents of the University of California.  All rights reserved.
- *
- * This code is derived from software contributed to Berkeley by
- * Chris Torek.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- * 1. Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- * 2. Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in the
- *    documentation and/or other materials provided with the distribution.
- * 3. Neither the name of the University nor the names of its contributors
- *    may be used to endorse or promote products derived from this software
- *    without specific prior written permission.
- *
- * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
- * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
- * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
- * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
- * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
- * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
- * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
- * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
- * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
- * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
-#include <stdio.h>
-#include <errno.h>
-#include <limits.h>
-#include "local.h"
-
-/*
- * ftello: return current offset.
- */
-off_t
-ftello(FILE *fp)
-{
-	fpos_t pos;
-
-	if (fp->_seek == NULL) {
-		errno = ESPIPE;			/* historic practice */
-		pos = -1;
-		goto out;
-	}
-
-	/*
-	 * Find offset of underlying I/O object, then
-	 * adjust for buffered bytes.
-	 */
-	FLOCKFILE(fp);
-	__sflush(fp);		/* may adjust seek offset on append stream */
-	if (fp->_flags & __SOFF)
-		pos = fp->_offset;
-	else {
-		pos = (*fp->_seek)(fp->_cookie, (fpos_t)0, SEEK_CUR);
-		if (pos == -1)
-			goto out;
-	}
-	if (fp->_flags & __SRD) {
-		/*
-		 * Reading.  Any unread characters (including
-		 * those from ungetc) cause the position to be
-		 * smaller than that in the underlying object.
-		 */
-		pos -= fp->_r;
-		if (HASUB(fp))
-			pos -= fp->_ur;
-	} else if (fp->_flags & __SWR && fp->_p != NULL) {
-		/*
-		 * Writing.  Any buffered characters cause the
-		 * position to be greater than that in the
-		 * underlying object.
-		 */
-		pos += fp->_p - fp->_bf._base;
-	}
-out:	FUNLOCKFILE(fp);
-	return (pos);
-}
-
-long
-ftell(FILE *fp)
-{
-	off_t offset = ftello(fp);
-	if (offset > LONG_MAX) {
-		errno = EOVERFLOW;
-		return (-1);
-	}
-	return ((long)offset);
-}
diff --git a/tests/stdio_test.cpp b/tests/stdio_test.cpp
index d054bf5..31acfec 100644
--- a/tests/stdio_test.cpp
+++ b/tests/stdio_test.cpp
@@ -1068,3 +1068,28 @@
   ASSERT_EQ(-1, fileno(stdin));
   ASSERT_EQ(EBADF, errno);
 }
+
+TEST(STDIO_TEST, fseek_ftell_unseekable) {
+#if defined(__BIONIC__) // glibc has fopencookie instead.
+  auto read_fn = [](void*, char*, int) { return -1; };
+  FILE* fp = funopen(nullptr, read_fn, nullptr, nullptr, nullptr);
+  ASSERT_TRUE(fp != nullptr);
+
+  // Check that ftell balks on an unseekable FILE*.
+  errno = 0;
+  ASSERT_EQ(-1, ftell(fp));
+  ASSERT_EQ(ESPIPE, errno);
+
+  // SEEK_CUR is rewritten as SEEK_SET internally...
+  errno = 0;
+  ASSERT_EQ(-1, fseek(fp, 0, SEEK_CUR));
+  ASSERT_EQ(ESPIPE, errno);
+
+  // ...so it's worth testing the direct seek path too.
+  errno = 0;
+  ASSERT_EQ(-1, fseek(fp, 0, SEEK_SET));
+  ASSERT_EQ(ESPIPE, errno);
+
+  fclose(fp);
+#endif
+}